Steam Assisted Gravity Drainage (SAGD) is an enhanced oil recovery technology for recovering heavy crude oil and bitumen. It is an advanced form of steam stimulation in which two parallel horizontal wells (one above the other) are drilled into the oil reservoir. High-pressure steam is continuously injected into the upper well which causes heated oil to gravity drain into the lower well and then pumped to the surface.
One of the challenges of SAGD is generating high quality steam. Because large amounts of water are needed for SAGD, water fed through the SAGD system is often recovered and recycled back as feedwater for additional steam generation. As expected, the recycled water can have many types of contaminants (suspended clays, free oil, dissolved organics, inorganics, etc.). Under certain conditions of temperature, pressure, and velocity, these contaminants will cause fouling in the heat exchanger and steam generator tubes, ultimately leading to steam generator failure. These interruptions to steam generations halt production.
Once-through steam generator (OTSG) is a type of steam generator that is used to generate the high quality steam needed for SAGD. Referring to FIG. 1, OTSG 10 includes a large, winding tube 20 in which feedwater is supplied at one end (inlet) and wet steam is produced at another end (outlet). Although the tube is continuous, it may be described as having an economizer section A (closest to the inlet), a superheater section C (closest to the outlet), and an evaporator section B located between the economizer and superheater sections. In the economizer section, temperature of the water is elevated close to the boiling point. Once the water reaches the evaporator section, it is converted into saturated steam. Lastly, the saturated steam is converted to superheated steam in the superheater section. A conventional OTSG boiler typically operates at around 80% steam and 20% blowdown water. The blowdown water contains salts and silica found in the water and produced from the reservoir.
Contaminants must be regularly removed from OTSG to ensure fouling does not occur frequently. Treatment of feedwater is needed to remove contaminants and protect steam-generating equipment. If contaminants are not removed, they can form solid masses that result in scale formation, fouling, and corrosion, among other problems. Precipitation of contaminants can deposit thermally insulating layers on heat exchange surfaces, causing boiler metals to eventually reach failure temperatures.
Piggings can physically remove solid contaminants from the OTSG. During low fouling periods, piggings may not be needed for several months. During high fouling periods, piggings may be required every month or so, leading to frequent costly interruptions in production. Each pigging event can easily cost millions of dollars.